Studies of inter- and intramolecular donor-acceptor interactions: Conformational population study of some substituted N-aralkyl-4-cyanopyridinium ions by NMR spectroscopy

Abstract

The relative orientation is studied (by NMR spectroscopy) of the substituted phenyl group (D) and the 4-cyanopyridinium group (A) in compounds of the type: DCH 2A and DCH(R)CH(R′)A with R,R′ = H or alkyl. For DCH 2A it is shown that ortho-alkyl substituents in D probably cause the molecule to adopt a conformation in which the ring-planes of D and A make a dihedral angle of 90°. From the measurement of vicinal coupling constants between protons attached to the central CC bond of DCH(R)CH(R′)A, the contribution of rotamers in which D and A are mutually gauche or trans has been calculated. The nature of R and R′ strongly influences the rotameric distribution, and consequently the average distance between the centers of D and A. It is suggested that for D = p-anisyl the repulsion between D and A in a gauche position is smaller than between two phenyl groups in such a position.